1. Field of the Invention
This invention relates to an incremental measuring system, particularly to a system for measuring lengths, comprising a scale member, which is provided with a scale, which is adapted to be scanned in a non-contacting manner, a scanning unit including scanning sets for scanning said scale and for generating analog measured-value signals in response to the scanning of the scale, and an evaluating unit for deriving countable signals from said measured value signal. Said scale member is provided in association with said scale with reference marks and the scanning unit comprises an additional scanning set for scanning said reference marks. An evaluating circuit is provided, which in response to the signals generated in response to the scanning of the reference marks generates reference pulses, each of which is associated with a location which is spaced a predetermined distance from one of the reference marks.
2. Description of the Prior Art
DE-A-34 45 694 discloses an incremental measuring system in which the scale can be optoelectronically scanned in a non-contacting manner by the scanning unit. Other known measuring systems are provided with scales which can be scanned by inductive, magnetic or capacitive methods. The measured-value signals generated in response to the scanning of the scale are periodic, e.g., sinusoidal analog signals, in which a complete signal period is associated with a complete scale increment, which in case of an optoelectronic scanning may consist of a bright field and a dark field of the scale. At least two measured-value signals are generated, which are displaced 90.degree. in phase and one of which leads the other in dependence on the direction of movement of the scanning unit so that that sequence can be used to determine the direction of the scanning movement. In the processing of the analog measured-value signals to generate digital countable signals the scale increments may be subdivided electronically or arithmetically, e.g., by means of microcomputers. The digital countable signals may be used as control signals for a machine or robot and for a display of the results of measurement by display units. In order to establish a definite relationship between each countable signal and the currently scanned location of the scale the counting means must be set to certain values, such as zero, when the scale is scanned at predetermined or preselectable locations. For that purpose, reference marks may be provided for permitting a generation of reference pulses in various length portions of the scale member in definite association with the scale. In that case a reference point which is being approached must be identified so that the control can be related to the associated location of the scale. It is known to provide expensive control and switching systems for a distinction between selected and unselected reference points. For exact measurements the reference points are defined by reference marks, which are provided on the scale member jointly with the scale but in a separate track. In applying said reference marks, care must be taken to ensure an exact register of the reference marks with increments of the scale. The scanning of such reference marks through an associated grating of a separate scanning unit will directly result in a generation of reference pulses, which are required to occur in proper edge coincidence and phase coincidence with the measured-value signal and countable signals. It is possible to provide a plurality of reference marks on the scale member and to select one or some of said reference marks for the actual control. That selection may be effected in that unselected reference marks are covered or removed. Such covering or removal of reference marks will be expensive if the measuring system is encapsulated, as is usually the case, and may be performed only by skilled labor and reference marks which have been removed cannot be renewed. Known selecting systems include mechanical on-off switches, which are known from DE-A-18 14 785 and are actuated by separate stops during the scanning movement of the scanning unit and which activate the evaluating circuitry only in response to the scanning of selected reference marks. In a design disclosed in DE-B-25 40 412 those mechanical switches have been replaced by magnets, which are adapted to be attached to the scale member at predetermined locations and serve to actuate selecting switches, which consist of reed relays provided in the scanning unit. It is also known to provide the reference marks in a plurality of reference tracks, to provide a scanning unit for each track, and to select a reference mark in that the scanning unit of the track in which that reference mark is contained is selectively enabled to generate reference mark signals. That concept requires an expensive design of the scale member and of the scanning units and requires difficult adjusting operations to be performed when the measuring system is to be activated. In preferred designs, the reference marks which may be selected are provided in a single track. In accordance with EP-A-0 239 768 an indentification of a selected reference mark is permitted in that the reference marks are evenly spaced apart and that reference mark which is adjacent to a selected reference mark is removed. Two scanning units are used, which comprise scanning sets, which are spaced like the reference marks and arranged in an antiparallel connection so that signals generated by existing reference marks will offset each other. When the selected reference mark is being scanned, the signal which would otherwise be generated by the adjacent reference mark is not generated because that adjacent reference mark has been removed. In response to the scanning of a selected reference mark, a reference signal which can be evaluated will be generated because in such arrangements it will be virtually impossible to effect a subsequent change of the selection of reference marks.
It is known from EP-A-0 172 323 to apply pairs of reference marks with a periodically changing spacing so that a length code is obtained, from which that half of a reference mark can be defined from which a reference pulse that is to be evaluated can be derived.
It is known from DD-A-58 394 to provide a scale member with code marks which can be displaced along the scale member and can be fixed in position and which can be scanned for a generation of control commands. It is not possible to position a code mark so accurately relative to the scale that the control signals which are generated are in phase and edge coincidence, like the reference pulses derived from stationary reference marks, with an analog measured-value signal generated in response to the scanning of the scale and with a digital countable signal which has been derived from that analog signal. That inaccuracy may result in measurement errors and/or control errors also in dependence on the directions in which a given reference mark is approached.